PROJECT SUMMARY The threat of a nuclear attack or disaster resulting in mass casualties is of increasing concern. Radiation injury frequently occurs in combination with thermal burns. Combined radiation and thermal burn injury (CRBI) results in impaired wound healing, exacerbated symptoms, and synergistic increases in mortality. Countermeasures that synergistically address the complex pathophysiology of CRBI at both local and systemic levels remain an unmet need. Gap junctions and their connexin (Cx) components are indispensable in mediating the cellular coordination required for tissue and organ development, repair, and homeostasis. Based upon this, a small synthetic peptide aCT1, comprising the carboxy-tail of Cx43, which regulates Cx43 function, was developed. aCT11 is a systemically stable truncated form of aCT1. A stable, easy-to-use, topical formulation of aCT1 (Granexin) has been successfully advanced through extensive preclinical work and three Phase 2 human clinical trials with clinically meaningful outcomes for scar reduction and the treatment of chronic wounds. Granexin is currently in Phase 3 trials for the treatment of chronic skin wounds. Preclinical studies further support clinical opportunity for aCT1, where daily Granexin treatment prevented injury progression and optimized clinical outcome of thermal burn injuries and acute high dose cutaneous radiation injuries. Cx43 also has key roles in maintaining intestinal homeostasis and intestinal mucosal repair and early efficacy studies in gastrointestinal-targeted murine partial body irradiation models support aCT11 in reducing mortality. Current guidelines in the evaluation of radiation countermeasure support the stepwise translation of acute radiation models to CRBI models and our exciting results support the strategy of synergistically using a topical (Granexin) and systemically delivered Cx43-based peptide (aCT11) in the mitigation and treatment of CRBI post-exposure. In response to the priorities of the Radiological/Nuclear Medical Countermeasure Product Development Program, our strategy is to develop a broad activity therapeutic strategy that can be readily administered in a mass casualty emergency scenario post-exposure and addresses both the cutaneous and systemic symptoms that synergistically result in CRBI-associated morbidity and mortality. The objectives of this Phase I SBIR are to i.) develop and fully characterize a murine model of combined partial body radiation and thermal burn injury and ii.) conduct a proof of concept preclinical efficacy study to evaluate in vivo therapeutic potential and mechanism of action of a combinatorial therapeutic approach involving topical Granexin and systemically delivered aCT11 in the treatment of CRBI. These studies will lay the foundation for large animal models that will serve as an integral part of an IND submission to the FDA. We hypothesize that application of this two-pronged approach will synergistically treat CRBI by effectively reducing the lethality and improving clinical outcome. Furthermore, these studies will validate a combined partial body radiation and burn injury animal model and provide insight into disease biomarkers and additional translational opportunity.